Large volumes of natural gas (i.e., primarily methane) are located in remote areas of the world. This gas has significant value if it can be economically transported to market. Where the gas reserves are located in reasonable proximity to a market and the terrain between the two locations permits, the gas is typically produced and then transported to market through submerged and/or land-based pipelines. However, when gas is produced in locations where laying a pipeline is infeasible or economically prohibitive, other techniques must be used for getting this gas to market.
A commonly used technique for non-pipeline transport of gas involves liquefying the gas at or near the production site and then transporting the liquefied natural gas to market in specially-designed storage tanks aboard transport vessels. The natural gas is cooled and condensed to a liquid state to produce liquefied natural gas (“LNG”). LNG is typically, but not always, transported at substantially atmospheric pressure and at temperatures of about −162° C. (−260° F.), thereby significantly increasing the amount of gas which can be stored in a particular storage tank on a transport vessel. Once an LNG transport vessel reaches its destination, the LNG is typically offloaded into other storage tanks from which the LNG can then be revaporized as needed and transported as a gas to end users through pipelines or the like. LNG has been an increasingly popular transportation method to supply major energy-consuming nations with natural gas.
Currently, all LNG transfer to and from LNG carrier ships at terminals is conducted with alongside, above water surface, cryogenic loading arms (“hard arms”), which consist of counterweighted hard-piped elements connected by swivels. A special connector, with emergency disconnect capability, is located at the end of the loading arm. This connector mates with the flange on the LNG carrier cargo manifold, typically located near amidships on the LNG carrier. As the LNG carrier must be moored alongside the berth to enable the loading arms to connect at the cargo manifold, the arrangement is known as alongside offloading/loading.
For offshore terminals, alongside, above water surface, offloading is typical. Marine versions of LNG loading arms are being designed with special swivels, stronger structural members, and specialized end connectors with targeting systems, all to enable connection and subsequent LNG transfer offshore. In these cases, the LNG carrier is typically moored alongside the terminal berth with nylon mooring lines and fenders to prevent damaging contact between the ship and berth structures. Although there are technical and operational aspects that require continued evaluation, marine alongside offloading is considered to be an extension of conventional LNG transfer technology.
In mild and moderate environments, offshore alongside offloading can achieve acceptable operability. However, as the severity of the environment increases, the portion of time that alongside berthing and offloading can take place decreases. Limiting factors include LNG carrier mooring line tensions and tug boat capabilities, as well as loading arm limits.
Alternatively, systems for transferring natural gas (in the gaseous state) through turrets, risers and subsea gas pipelines have been designed, licensed for operation, and are being built. Examples include the Excelerate “Energy Bridge” and Leif Hoegh/Hamworthy's Ship Regasification Vessel (SRV) concepts. The Energy Bridge and SRV concepts use disconnectable turrets. Additional background can be found in U.S. Pat. No. 5,983,931 to lngebrigtsen et al., U.S. Pat. No. 5,025,860 to Mandrin, U.S. Pat. No. 5,878,814 to Breivik et al., U.S. Pat. No. 6,003,603 to Breivik et al., U.S. Pat. No. 6,517,290 to Poldervaart, U.S. Pat. No. 6,546,739 to Frimm et al., WO 2004/080790 to Korsgaard, G.B. 2,382,809 to de Baan, WO 93/24733, WO 93/24732 to Breivik et al., US 2002/174662 to Frimm et al., U.S. Pat. No. 5,651,708 to Borseth et al., U.S. Pat. No. 5,697,732 to Sigmundstad at al., FR 2 770 484 (Doris Engineering), U.S. Pat. No. 5,305,703 to Korsgaard at al., WO 02/092423 (Ingenium AS; Fosso, Jan), U.S. Pat. No. 5,339,760 to Korsgaard et al., and U.S. Pat. No. 5,628,657 to Breivik et al.
Due to the increase in LNG demand seen in recent years, increased emphasis has been placed on cost, design and schedule efficiency of new LNG transfer projects in order to reduce the cost of the delivered gas. Improvements in cost, design, and schedule efficiency can help mitigate the substantial commercial risk associated with large LNG transfer projects.